Again: we tried several tools including Balsamiq, Mockingbird, Photoshop, PowerPoint, Keynote, and others. Ultimately, what worked best was a combination of a whiteboard and paper.
1) Gather a few team members in a meeting room
2) Give each person their own color marker
3) Go nuts on the whiteboard, draw whatever comes to mind.
4) Make lots of revisions. This is the best part: It's a whiteboard! Erase with your finger. Get over it, get dirty, mix colors, make a mess. Who cares?
5) When you're ready, let your best "artist" redraw it on the whiteboard nice and big so everyone can make even more revisions.
6) On paper or a laptop or whatever, take notes of any edge cases or things you simply didn't think of earlier.
7) Repeat 3 to 6 until you're happy.
8) Transcribe to paper. This is vital! Do not just take a photo. You need to have every single screen on it's own piece of paper, so you can thumb through them later.
9) Write the name of the screen on the top edge of the back of the page. This is so you can quickly skim through the stack backwards to find the page you are looking for.
10) Write any notes about the behavior of the page on the front as callouts or on the back as commentary.
11) Photocopy the whole stack and hand out to the team.
12) Encourage everyone else to write all over them in red pen
13) Bring the notes to the next meeting & start all over until it's close enough to go to implementation.
Note: this advice is for NEW UIs. I still haven't quite figured out the best way to handle changes to EXISTING UIs. There seems to be a mismatch in our brains when you try to draw stuff on top of print outs of real screens. The resulting markups look out of place next to polished features, so they don't get the same thought process as a completely hand drawn mockup. I've got to do more experimentation here...